Certain switch disconnectors, for example ABB Kraft AB's switch disconnector of type NAL/NALF designed for medium voltage, have an operating spring in the form of a torsion spring, which gives the contacts of the disconnector a rapid movement upon opening and closing of the disconnector. The operating spring is mechanically connected to an operating shaft for tensioning the spring and to the contacts of the disconnector. In one embodiment of this disconnector, the spring is tensioned with the aid of an operating device connected to the operating shaft until a dead center is passed, whereby opening and closing, respectively, take place. In another embodiment, the operating spring is tensioned but the movement is stopped after passage of the dead center. The relevant operation can then be released with the aid of, for example, a release magnet.
Prior art motor-operated devices have a screw driven by an electric drive motor by means of a gear comprising a worm gear. A nut is moved axially along the screw upon rotation thereof, and the nut is connected to the operating spring of the disconnector via a lever mechanism. It has proved that the screw is subjected to large axial forces and that large frictional forces arise in the mechanism. This makes possible a strong construction of the operating device and a drive motor with such a high power that, for economical reasons, an ac motor is required. The mechanism is also relatively complicated. These factors result in a high manufacturing cost. The high motor power also increases the costs of control and switching means for the drive motor of the operating device and make it difficult or impossible to provide battery stand-by operation of the operating device in an economically reasonable way.
From DE 311 47 27, a spring-operated operating device is known, the object of which is to provide a simple and space-saving device where a drive spring common to both directions carries out the operating movement rapidly. The operating device comprises a motor-driven drive pulley which, via a pin, drives a first cam disk in which the operating spring is arranged. The first cam disk drives, via a pin, a second cam disk which is connected to an electric switching device. The drive pulley engages with the first cam disk after a rotation of about 90.degree., whereupon the drive pulley rotates the first cam disk with the drive spring applied thereon to an upper dead-center position at about 180.degree., in which the first cam disk engages with the second cam disk. When the dead-center position has been passed, the cam disks continue the movement of the force from the drive spring, whereby switching takes place. During each operation, the drive pulley and the first cam disk move one full turn, which means that a manual operation of an electric switching device with one closing and one opening direction cannot be carried out. A disadvantage of the known device is that three discs are needed to make possible one operation. A further drawback with the device is that an operating spring is used and that this must be pre-stressed by a motor. The spring must therefore be oversized by about 20% to give sufficient safety and to overcome friction losses. The motor, in turn, must be oversized by about 20% to be able to prestress the operating spring in a reliable manner.
One further disadvantage of these prior art operating devices is that they have insufficient flexilibity during mounting, that is, that they cannot be mounted in an arbitrary manner in relation to the disconnector, for example on an optional side of the disconnector.